Study On Microstructure And Properties Of Martensitic Stainless Steel Fabricated By Laser Melting Deposition

Remanufacturing is the process of specialized repairmen and upgrade of failed parts,and performance of repaired components meet or even exceed the level of original productions.What’s more,remanufacturing can save cost by 50%,energy by 60%,materials by 70%,and greatly reduce adverse impact on environment.As a national emerging strategic industry,remanufacturing conform to the concept of green and intelligent manufacturing proposed by《Made in China 2025》,and is highly valued by government and enterprises.Laser melting deposition(LMD)is one of the key technologies for realizing remanufacturing and rapid prototyping with its high energy density and processing precision,wide material selection and good metallurgical bonding strength.At present,laser melting deposited remanufacturing usually adopt lower laser power and smaller laser spot,resulting in low repair efficiency,and the repair layer is mostly single-layer.Hence,it is difficult to achieve high-performance repairment with multilayer deposition.Moreover,the powders used for laser melting deposition are generally from thermal sprayed materials.Lack of specific powders for laser melting deposition limits LMD application fields.In view of the above problems,this thesis focuses on the research of laser melting deposited martensitic stainless steel for realizing remanufacturing of sprocket and experiments were conducted with a large-spot and high-power semiconductor laser.The microstructure and properties of the deposited layer were systematically studied from in three aspects,i.e.process,powder composition and heat treatment.The main research work of this thesis is as follows:(1)This thesis studied the influence of process parameters on the formability of the deposited layer,the microstructure of heat affected zone and depisiton,as well as hardness of the deposited layer.Moreover,the strengthening mechanism of the martensitic stainless steel deposition was revealed.A well-formed layer was obtained,with heat affected zone width of approximately 2mm.The hardness of the deposited layer was 420-592HV,which was mainly contributed to phase transformation and solid solution strengthening,and microhardness of deposition increased with the increase of laser energy density.(2)Fe-Cr-B and Fe-Cr-B-Nb alloy powders were developed and the influence of Nb on the microstructure and properties of martensitic stainless steel deposits was investigated.The results show that Nb promotes segregation of carbon in interdendritic area and increases the strength of the deposited layer,but decreases the hardness and wear resistance.Moreover,Nb can improve the corrosion resistance of martensitic stainless steel deposits.(3)This thesis explore evolution process of internal defects and microstructure of multilayer deposition.The main body of deposition is fine and uniform dendrites,and the grain morphology is diversified at the overlap of the layers.The remelting of the deposited layer reduced the number of cracks,but increase the pore content.A quantitative measurement method for the interfacial bonding strength was designed and applied to measure bonding strength of deposition.The results show that when the deposition layer height is small,the interfacial bonding strength gradually increases with the increase of the deposition layer number.(4)The effect of annealing temperature on the microstructure and hardness of the deposited layer was studied.There is a carbon-rich zone at the interface of deposition and substrate,and when the annealing temperature is 500~oC,carbon segregation is most severe.When the annealing temperature is 700~oC,the carbon-rich zone disappears,and the carbon content of the deposit layer begins to be higher than that of the substrate.Meanwhile,the martensite decomposite and microhardness decrease to 390HV.Under the annealing temperature of 700~oC,the content of reverse austenite reached maximum,and the dominant strengthening mechanism consists of secondary phase strengthening and fine grain strengthening.Under the annealing temperature of 900°C,microstructure was relatively homogeneous,but boron segregation still existed.